The present invention relates to efficient encoding systems for efficiently coding video signals into a smaller amount of coded data on various types of system for recording, transfer and display the video signals. Particularly, the present invention relates to interframe/interfield predictive encoding systems with respect to moving images.
Moving images have a strong interframe correlation and therefore, interframe prediction coding in which interframe prediction is performed to generate prediction errors and those prediction errors are coded, is effective for coding video signals of moving images. However, recursive prediction is conventionally performed with respect to interframe predicative coding and so decoding a current frame necessitates having all of the past decoded video signals.
On one hand, for storage media such as VTR and video disks and the like, it is required to have special playback such as random access, high-speed search and reverse playback. Thus, when interframe predictive coding is used for those media, it is necessary to have independent frames which are to be coded periodically by intraframe coding without interframe prediction. Coding methods for effectively increasing the coding efficiency of those independent frames and facilitating reverse playback has been disclosed in U.S. Pat. No. 4,985,768 of the same inventor of the present invention. Furthermore, Japanese Patent Application No. 1991-77625, also of the same inventor, discloses a coding method in which the coding method disclosed in this U.S. Patent is applied to interlace signals.
FIG. 1 is a block diagram that shows a simplification of the configuration of the interframe predictive encoding system shown in FIG. 7 of the U.S. Patent described above. In FIG. 1, the (N-1) frame memory 31 of FIG. 7 of the U.S. Patent is shown as a 3-frame memory 2 with N=4. The prediction error subtractor 2, coefficient multipliers 34 and 35, adder 36 of FIG. 7 are combined and shown as a predictor 3 in FIG. 1. Furthermore, the orthogonal transform device 3, quantizer 4 and variable-length encoder 5 of FIG. 7 are combined and shown as an intraframe encoder 4 in FIG. 1. In FIG. 1, the switch 40 of FIG. 7 of the U.S. Patent is omitted, and a synchronizing signal separator 8 and frame counter 9 are newly added.
In FIG. 1, an incoming video signal of moving images is inputted to a switch SW11 and the synchronizing signal separator 8 via an input terminal 1. The synchronizing signal separator 8 separates vertical synchronizing signals from the video signal and supplies the synchronizing signals to the frame counter 9. This frame counter 9 counts the number of frames and that count output causes the switch SW11 and a switch SW12 to be switched. When an independent frame of the video signal is inputted from the input terminal 1, the switches SW11 and SW12 are both switched to a terminal a, and when a dependent frame of the video signal is inputted from the input terminal 1, the switches SW11 and SW12 are both switched to a terminal b. When both the switches SW11 and SW12 are switched to the terminal a, the video signal that is inputted to the input terminal 1 is inputted to the intraframe encoder 4 via the switches SW11 and SW12 and is coded. Then, a coded video signal is outputted from a data output terminal 5. The incoming video signal is also inputted to a frame memory 6 and stored there. Then, a video signal that has already been stored in the frame memory 6 is inputted to a frame memory 7 and stored there.
After this, when both the switches SW11 and SW12 are switched to the terminal b, the incoming video signal that is inputted to the input terminal 1 is inputted to the 3-frame memory 2 via the switch SW11, where the signal receives a time delay of four frames. When an independent frame is inputted from the input terminal 1, the content of that frame is held by the frame memory 6 and so in the 3-frame memory 2, a 4-frame delay is performed with 3-frame capacity. In the predictor 3, a signal that has this time delay is predicted by weighting on the basis of the number of frames from two independent frames and a prediction error is generated. This error is intraframe-coded by the intraframe encoder 4 and the coded video signal is outputted from the data output terminal 5.
In the encoding system, if N=4, then it is necessary to have memories of five frames for the 3-frame memory 2 and the frame memories 6 and 7.
FIG. 2 comprising FIGS. 2(a)-2(d) is a view showing the timing of the interfield predictive coding method disclosed in Japanese Patent Application No. 1991-77625 described above. What is shown here is a coding method for interlace signals for storage media. Respective divisions in the figure are fields, and upper portions of (a) through (c) are odd-numbered fields, and lower portions of (a) through (c) are even-numbered fields. As shown in the figure, the odd-numbered fields and the even-numbered fields are mutually displaced by the time of 1/2 of a frame. In FIG. 2, independent fields (indicated by IF in FIG. 2(a)) are coded by odd-numbered field intervals (5, in this case) and dependent fields between them are predicted from the independent fields located anterior and posterior to the dependent fields. Prediction is made from two independent fields or three independent fields located anterior and posterior to the dependent fields. In the case of the prediction method for a field to be predicted being an odd-numbered (even-numbered) field, that uses the three independent fields, prediction is performed from two odd-numbered (even-numbered) independent fields and one even-numbered (odd-numbered) independent field. This situation is shown in FIG. 2(a).
FIG. 2(b) shows which independent fields are used for performing the prediction. A, B and C show the independent fields. Four dependent fields surrounded by bold lines are predicted by the independent fields inside parentheses. FIG. 2(c) shows the correspondence between the center fields of the three independent fields that are used for the prediction, and the lower case letters which indicate dependent fields to be predicted from the three independent fields, with numbers indicating input order. More specifically, b1, b2, b3 and b4 are predicted from the three fields A, B and C. Then, FIG. 2(d) shows input order of fields with the dependent fields being predicted at different timings.
According to the coding method shown in FIG. 2, it is necessary to have memories for 11 field portions because of four fields for the independent fields, and the seven fields (9-field delay) for the delay of the dependent fields.
In the interframe/interfield predictive coding system of U.S. Pat. No. 4,985,768 and Japanese Patent Application No. 199-77625, there is the problem that the production cost of the system becomes higher because it is necessary to have many frame (field) memories on the interframe (interfield) prediction in a reverse direction necessary for reverse playback and the like in a VTR.